HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), also referred to as octogen or cyclotetramethylenetetranitramine, is a highly energetic material that is useful in various explosives and propellants for military and non-military applications. HMX is recognized as one of the most powerful nitramine explosives, and is used as the benchmark for all other explosives.
HMX is known to exist in four different crystal structures or polymorphic forms--alpha, beta, gamma and delta. Of these polymorphs, it was long believed that the beta form was the least sensitive and most stable, and thus the beta polymorph has been the most widely used form of HMX. The alpha and gamma polymorphs have commonly been dismissed as too dangerous for use due to greater sensitivity, and the delta polymorph is so unstable that it is of no commercial significance.
Despite its superior energetic properties, HMX has not been widely used as an explosive due to difficulties in large-scale production and excessive manufacturing costs. The first known process for the manufacture of HMX, the Bachmann process, was developed in the 1940's. The Bachmann process involves nitrolysis of hexamine (also known as hexamethylenetetraamine) with a mixture of nitric acid and a large excess (e.g., 20-fold) of acetic anhydride. HMX is produced as a by-product or contaminant along with a greater amount of another explosive, RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine). The Bachmann process typically provides yields of 80-84%, of which only about 10-40% is HMX, based on the methylene content of the feed. When fully optimized for HMX, the maximum reported yield of HMX per mole of hexamine feed is about 64%. Due to the inefficiencies in the process, and the large amounts of hazardous waste materials produced, it is not appropriate for large-scale industrial production.
Other synthetic routes for making HMX have been proposed, involving various intermediates. One such intermediate that has been used to produce HMX is DAPT (3,7-diacetyl-1,3,5,7-tetraazabicyclo-[3.3.1]-nonane). DAPT is generally made by reaction of wet hexamine and acetic anhydride. One problem common to all methods of manufacturing DAPT is the massive amount of heat generated by the reaction. Because DAPT in solution will decompose rapidly at temperatures ranging from about 20-120.degree. C., depending on pH, it is necessary to remove heat from the reaction mixture and thus keep the temperature low. In effect, the rate of DAPT production is typically limited by the capacity of the reaction apparatus to withdraw heat by means of heat exchangers or the like. Due to the extremely exothermic nature of this reaction, in practice the rate of addition of acetic anhydride to the hexamine has been kept very low, so the rate of heat generation is kept at manageable levels. As a result, the time required to synthesize a given amount of DAPT is quite long, and the cost is relatively high.
One method proposed for dealing with the tremendous amounts of heat generated by the reaction is to mix ice and water with hexamine to create a slurry, and then add acetic anhydride to the slurry. (Lukasavage U.S. Pat. No. 5,246,671.) Suitable temperatures for this reaction slurry are described as ranging from -18.degree. C. up to 120.degree. C.
Another intermediate that can be used in the production of HMX is TAT (1,3,5,7-tetraacetyl-1,3,5,7-tetraazacyclooctane, also known as 1,3,5,7-tetraacetyloctahydro-1,3,5,7-tetrazocine). TAT can be prepared by heating DAPT with acetic anhydride under anhydrous conditions, but the yields from this process have been poor. Another process used to prepare TAT involves reacting DAPT with acetic anhydride, acetyl chloride, and an alkanoic acid salt such as sodium acetate, under anhydrous conditions. (Siele U.S. Pat. No. 3,979,379.) However, this process uses a large excess of acetic anhydride, thus making it relatively expensive. Yet another process that has been used to make TAT involves reacting DAPT with acetic anhydride in the presence of a metal acetate under anhydrous conditions at temperatures of 100-125.degree. C. (Surapaneni U.S. Statutory Invention Registration H50.) However the reaction conditions and yield that have been reported for this process indicate that it is not economical for commercial use.
HMX can be synthesized by nitrolysis of TAT, using nitric acid and dinitrogen pentoxide or phosphorous pentoxide, at temperatures ranging from room temperature up to 40.degree. C. (Lukasavage U.S. Pat. Nos. 5,124,493 and 5,268,469.) This process too, however, has not seen acceptance on a large production scale due to the economics involved.
SOLEX (1-(N)-acetyl-3,5,7-trinitro-cyclotetramethylenetetramine) is another nitramine explosive, which is a by product of the nitration of TAT to form HMX. SOLEX is relatively stable, having twice the impact resistance of RDX, is easily isolated, and can be produced using far less nitrating agent than is required for the direct preparation of HMX from TAT.
One process that has been described for the production of SOLEX involves adding TAT to a solution of 98% nitric acid and phosphorus pentoxide at a temperature between 20-45.degree. C. (Lukasavage U.S. Pat. No. 5,120,887.) The purity and product yields from this method are reported to be quantitative. Significantly, however, this method requires an excess of nitrating agent, i.e., 7.5 grams of nitric acid per gram of TAT used, which makes the process relatively expensive. The SOLEX can be converted to HMX by treatment with strong nitric acid.
Beta-HMX has been widely used as an explosive, despite the difficulties and expense involved in its manufacture. One specific form that is sold is referred to as Class 5 beta-HMX (defined as particulate beta-HMX of which 98% by weight will pass a 325 mesh (44 .mu.m) sieve). Class 5 beta-HMX can be sold for a higher price than coarser beta-HMX products, but is also more difficult to make. Usually it is made by first forming larger beta-HMX particles, and then either grinding them in a water slurry or "sand blasting" them against a hard surface, whereby the desired finer beta-HMX particles are produced. This procedure is troublesome and relatively expensive.
Recently it was discovered that alpha-HMX can be produced that exhibits less sensitivity to impact than beta-HMX. (Lukasavage U.S. Pat. No. 5,268,469.) Production of this polymorph at a reasonable cost on a large scale would be advantageous as it would be useful as a substitute for the beta-HMX used in existing explosive formulations.
Another problem in the prior art involves making durable shaped articles that contain explosive materials. Such articles typically comprise both an explosive substance and a binder, the latter giving the composition the physical characteristics needed to retain the desired shape. However, such binders or other additives dilute the explosive power.
A long-standing need exists for an improved process for making HMX, and improved HMX compositions and articles that exhibit desirable stability, impact sensitivity, and explosive properties. A particular need exists for an improved process for making alpha-HMX that is relatively impact-insensitive.